The genetic and physiological basis of various characters of Salmonella typhimurium (and related bacteria), including properties relevant to its pathogenicity, will be investigated, in particular the following: (a) Identification, mapping and test of function of genes determining structure of somatic lipopolysaccharide, especially rfa genes, for LPS core, and of genes for unidentified or partly identified non-LPS components of the outer membrane, forming the bacterial surface, especially those serving as bacteriocin or phage adsorption sites: effect of alteration of LPS core and other outer membrane components on permeability and on virulence. (b) Identification, mapping and test of function of genes regulating motility and chemotaxis, especially identification of genes for components (hook, units of basal body) of flagella: mechanism of phase-variation. (c) Investigation of genetic and physiological basis of alterations of host properties resulting from acquisition of R factors or other plasmids, especially the diminished virulence which sometimes accompanies acquisition of an R factor: and the increased (spontaneous) mutability, diminished UV sensitivity and increased UV mutagenic response conferred by some plasmids.